This is application is a continuation of U.S. application Ser. No. 11/786,060, filed on Apr. 10, 2007, now U.S. Pat. No. 7,928,664, which claims priority from U.S. application Ser. No. 60/790,627, filed on Apr. 10, 2006, both of which are incorporated by reference.
The present invention relates to illumination systems and methods, and more particularly, to illumination systems and methods for general lighting and commercial signs.
Commercial lighting systems, used on the outside of commercial buildings for advertising purposes, include channel letters. Channel letters generally include a housing with a concave cross-section about 5″ deep, made of aluminum or plastic. The housing cavity, shaped as the letter, is covered by a translucent plastic sheet of a selected color, illuminated by a light source mounted within. Neon and fluorescent lights provide suitable illumination, allowing the letters to shine brightly when turned on. However, fluorescent light sources have a relatively short life of approximately 20,000 hours. They operate at high voltage (for example, 7,000 to 15,000 volts for neon) and can consume a relatively large amount of electrical power. Furthermore, fluorescent light tubes are usually quite fragile. Still, fluorescent lights have been used for decades and decades in different fields.
Light emitting diodes (LEDs) are currently used for a wide range of applications, providing a variety of advantages relative to conventional lights, such as neon or fluorescent bulbs, due to their advantageous qualities. LEDs are compact, rugged, and consume less power, being 30 to 70% more energy efficient than conventional lights. LEDs have a relatively long life of up to 100,000 hours and operate at low voltages (4 VDC to 24 VDC).
FIG. 1 shows an example of an illuminating system as used presently for general lighting and commercial sign lighting systems. The illumination system includes a DC power supply usually connected to 120 VAC. The output from the power supply provides DC voltage (from 4 to 24 VDC) to a supply rail providing electrical connection to LED arrays arranged in several illumination modules. These modules are usually connected in parallel on a DC supply bus. The LEDs are connected with wires that are soldered permanently at a fixed spacing; that is, use fixed electrical wiring. Every single illumination module is connected to the next module using two or four wires (i.e., positive and negative inputs and outputs) by mechanically creating electrical contact. Each LED module uses a ballast resistor R (or regulator) to provide a constant current to the LEDs connected in series since LEDs operate with current (and not voltage). The modules are usually located inside a letter channel. This dissipative method normally uses as much energy in the ballast resistor (i.e., dissipated energy) as in the LEDs, resulting in efficiencies frequently lower than 50%. This means there is more energy wasted in heat than energy used by the LEDs to produce light.
The brightness of an LED depends upon the amount of electrical current flowing through the diode. However, while an increase in current increases the brightness of the light emitted by the LED, it also increases the connection temperature, which can decrease the LED's efficiency and life. Given that LEDs are often constructed of semiconductor materials that share many comparable properties with silicon and gallium arsenide, this can be highly detrimental. As a case in point, for every 10° C. increase in temperature, the useful life of silicon and gallium arsenide drops 2.5-3 times.
The conventional light circuits can be prone to problems other than those described above. The system of FIG. 1 includes modules joined with connectors, which are prone to reliability troubles. For example, connectors can fail due to corrosion, and many devices, as well as commercial lighting systems, are used outdoors. Also, while diodes are generally biased through a series resistor from a regulated voltage supply, the amount of current going through the diode depends also on the forward voltage drop over the diode, which drops with changes in its size, age, and its temperature at the time.
The LEDs have been also used as light sources in applications such as emergency EXIT signs. The EXIT signs contain a reflector in the rear, having a series of curved, concave surfaces shaped as letters and background area. The LEDs are mounted in the center of each surface to provide light that is projected outwardly.
There is still a need for an improved illumination system that is simple and quick to install and that operates at relatively high power efficiency.